Distinguishing the effect of diapir growth on magnetic fabrics of syn-diapiric overburden rocks: Basque-Cantabrian basin, Northern Spain

An analysis of Anisotropy of Magnetic Susceptibility was done on Aptian–Albian sediments from the Basque–Cantabrian basin. Samples were collected from 39 sites in the halokinetic sequences of the Bakio, Bermeo, Guernica and Mungia diapirs; 28 sites were sampled close to diapirs, and 11 sites were far from the diapir edges. The magnetic foliation is parallel to bedding, suggesting it reflects depositional and compaction processes, whereas the orientation of magnetic lineation varies. Far from the diapir edges, the magnetic lineation is interpreted as being related to the regional Pyrenean compression. Close to diapir edges, the observed behaviour shows that diapirs, predominantly formed by rigid ophites, have acted as buttresses, with shadow areas at their northern faces being protected from the Pyrenean compression. The high sensitivity of AMS makes it a very useful tool to distinguish deformation in halokinetic sequences related to diapir growth from that related to subsequent compression.Funding came from projects CGL2014-54118-C2-1-R and CGL2014-54118-C2-2-R (Spanish Ministry).Peer reviewe

Early-Middle Miocene subtle compressional deformation in the Ebro foreland basin (northern Spain); insights from magnetic fabrics

The results of anisotropy of magnetic susceptibility analyses of 19 sites carried out on magnetostrigraphically-dated, Lower to Middle Miocene (20.4 to 13.7 Ma) 'non-deformed' mudstones from the central part of the Ebro basin reveal the presence of a subtle tectonic overprint lasting at least until the Langhian (Middle Miocene) in the southern Pyrenean foreland. Magnetic ellipsoids show a sedimentary fabric in 42% of sites and a weak and well-defined magnetic lineation in 47% and 11% of sites. The magnetic lineation is roughly oriented around the east-west direction, compatible with a very weak deformation occurring there and related to the north-south compression linked to the convergence between Europe, Iberia and Africa during the Early-Middle Miocene. A slight variation of the magnetic parameters Km, Pj, and T exists through time, probably due to changes in the sedimentary conditions in the basin

La Magnetoestratigrafia y la Escala del Tiempo Geológico basada en las inversiones del campo magnético terrestre

El campo geomagnético ha experimentado multitud de inversiones de polaridad magnética no periódicas a lo largo de la historia de la Tierra. La magnetoestratigrafía se basa en la capacidad de las rocas de adquirir una magnetización remanente paralela a la dirección del campo magnético terrestre presente cuando éstas se forman, lo que permite establecer una zonación de las secuencias sedimentarias en función de la polaridad magnética. Una zonación magnetoestratigráfica permite dividir el registro estratigráfico en porciones de tiempo que se pueden correlacionar a lo largo y ancho del planeta. La datación magnetoestratigráfica hace referencia a la identificación en el registro sedimentario de 'magnetozonas', que pueden correlacionarse con crones geomagnéticos de edad equivalente

'Anomalous' magnetic fabrics of dikes in the stable single domain/superparamagnetic threshold

'Anomalous' magnetic fabrics in dikes that appear to indicate flowinto thewall confound many workers. Here, we present extensivemagnetic data on five dikes from Tenerife, Canary Islands, and use these to interpret the causes of the anomalous fabrics. Comparison of the anisotropy of magnetic susceptibility (AMS) and anhysteretic magnetization (AARM) results show that, in some cases, the anomalous fabrics are caused by single-domain grains, which produce AMS fabrics perpendicular to the grain elongation, whereas AARM fabrics are parallel. To check this, hysteresis experiments were used to characterize the domain state. These show most are mixtures of pseudo-single-domain or single-domain plus multi-domain particles, but many have wasp-waisted hysteresis loops, likely indicating mixed populations of stable single domain and superparamagnetic grains. First-order reversal curves were used to better characterize this and show mixtures of stable single-domain and superparamagnetic grains dominate the magnetic signal. Magnetic particles at the stable single-domain/superparamagnetic threshold are unstable at timespans relevant to the analytical techniques, so they produce complicated results. This suggests that anomalous AMS fabrics in dikes cannot simply be attributed to elongated stable single-domain particles and that mixtures of the different grain types can produce hybrid fabrics, in which the fabrics are neither perpendicular or parallel to the dike plane, that are difficult to interpret without extensive magnetic analysis

Datos paleomagnéticos en materiales sin-diapíricos Aptienses-Albienses (cuenca Vasco-Cantábrica, N Iberia)

In order to obtain a kinematic model linking the diapir formation and the geometry of the syn-diapir sediments, a paleomagnetic study has been conducted in selected syn-diapiric sequences of the Basque-Cantabrian Basin. The studied diapirs (Bakio, Bermeo, Guernika and Mungia) developed during the Early Cretaceous in relation to the North Iberian extensional margin which was subsequently reactivated during the Pyrenean contractional deformation (Late Cretaceous-Miocene). They are cored by Triassic red clays and evaporites and they are flanked by synkinematic Albian shelf and slope carbonates and Upper Albian to Cenomanian siliciclastics. The paleomagnetic study has focused on the synkinematic overburden to detect and quantify vertical axis rotations related to the growth of the diapirs. 29 paleomagnetic sites have been analyzed. After obtaining the site mean directions it can be concluded that most of the sites are remagnetized, hindering the kinematics of the diapirs growth to be deduced. The age of this remagnetization is difficult to assess, it could be either an earlier Albian-Maastrichtian remagnetization or a remagnetization linked to the Pyrenean compression.Trabajo financiado gracias a los proyectos coordinados CGL2014-54118-C2-1-R y CGL2014-54118-C2-2-R y al GGAC 2014SGR467Peer Reviewe

Petrological characterization of flint from Montmaneu's Limestones Formation (Eastern sector of the Ebro Basin)

Flint from Montmaneu's Limestones Formation of Rupelian age (Oligocene) is being here characterized. The analysis of the compositional texture, mineralogy and geochemical features has revealed this flint is highly pure and of microcrystalline texture, with a significant content of moganite. This flint is product of an early diagenetic replacement of limestone from littoral lacustrine facies. This flint was exploited by human populations in a regional range, especially in the early stages of the NeolithicSe caracteriza el sílex de la Formación calizas de Montmaneu, de edad Rupeliense (Oligoceno). El análisis textural, mineralógico y de sus rasgos geoquímicos indica que se trata de un sílex microcristalino, bastante puro, con una proporción significativa de moganita. Es producto del reemplazamiento diagenético temprano de calizas de facies lacustres litorales. Este sílex fue objeto de una explotación regional, especialmente en los primeros momentos del Neolític

Paleomagnetic constraints on the kinematics of inverted passive margins with salt diapiric structures. Preliminary results from syndiapiric overburden rocks of the northern margin of the Basque-Cantabrian extensional Basin.

Salt levels deposited prior to deformation in either passive margins or orogenic settings generate a decoupling in deformation, with differential deformation in layers above and below the salt detachment. Distribution of salt detachments controls differential shortening and formation of orogenic arcs; and widespread diapirism can generate isolated minibasins which may record vertical axis rotations during their development. Our study aims to unravel the kinematics of salt diapiric structures by means of a paleomagnetic study of a portion of the Basque Arc, in the northern margin of the Basque-Cantabrian extensional Basin, where the salt overburden appears compartmentalized by several salt walls (Bakio, Bermeo, Guernica and Mungia diapirs). In this setting, 65 paleomagnetic sites from the overburden sequence have been analyzed. Predominant anticlockwise rotations with some clockwise and anticlockwise rotations in opposing flanks of the salt walls are observed. Fold tests reveal that some of the sites are clearly remagnetized, whereas some other sites show a prefolding magnetization. These observations are confirmed by magnetic hysteresis and back-field experiments in part of the studied rocks. Although further analyses will be necessary to explain the origin and distribution of the remagnetization, a preliminary kinematic model for the suprasalt deformation together with the underlying decoupled autochtonous materials can be established

Remagnetización alpina en la Serra del Cadí (Pirineo Oriental)

The eastern Pyrenees, particularly the Cadi Range, displays the less density and quality of paleomagnetic data of the chain. This paper revisited one of the central sections (Pobla de Lillet) where we have resampled Permian, Maastrichtian and Paleocene rocks. Best results were attained in the Permian red sandstones and in the Cretaceous limestones. In the Permian, two components with paleomagnetic meaning were identified. The high temperature component (up to 670 ° C) is similar to that defined in pioneering works, it appears to be primary and does not show significant rotations. The intermediate temperature component (up to 500 ° C), not identified in previous works, seems to be a syntectonic remagnetization with Alpine age. The characterization of rotations and the extension of this remagnetization (as an additional source of kinematics information) should be addressed in Permian rocks in future works since the paleomagnetic signal of Cenozoic rocks is of lower qualityTrabajo financiado con los Proyectos Pmag3Drest-CGL2009-14214 y DR3AM- CGL2014-55118 del MINECO y del 3DR3- PI165/09 del Gobierno de Aragón.Peer Reviewe

Datación magnetostratigráfica de la molasa Surpirenaica (Fm Uncastillo, Oligoceno-Mioceno)

The Uncastillo Fm crops out along the southern boundary of the External Sierras, which represents the southernmost limit of the Pyrenean Range. Three tectosedimentary units related to tectonic pulses have been previously differentiated in this formation. The sedimentary evolution of the Uncastillo Fm is mainly related to the tightening of the WNW-ESE Santo Domingo anticline in the External Sierras during Chattian-Aquitanian times and records the younger tectonic movement of the south Pyrenean sole thrust. The Uncastillo Fm represents the change to overall southwardflowing alluvial and fluvial systems from overall west-northwest-flowing fluvial systems (Campodarbe Fm). Recent magnetostratigraphic results from the underlying Campodarbe Fm shift to younger ages (from chron 10r to 7r) the top of the Campodarbe Fm in the proximal area of the fluvial system (Luesia fan). In order to check that reassignment and refine the age of the Uncastillo Fm, a new magnetostratigraphic study has been conducted in laterally equivalent deposits that represent middle-distal areas located to the east of the Luesia fan. These deposits consist of mudstones with interbedded sandstones. These finer grain size sediments allow for a magnetostratigraphic sampling in the Fuencalderas section of ~1300 m, which complete the chronostratigraphic frame of the last tectonic pulses of the Pyrenean Range.Trabajo financiado por los proyectos SEROS (CGL 2014-55900-PL), DR3AM (CGL2015- CGL2014-55118) y Grupos Geotransfer y Análisis de Cuencas de la Universidad de Zaragoza. Es una contribución del Institut de Recerca Geomodels y del Grup de Geodinàmica i Anàlisi de Conques (2014SGR467).Peer Reviewe

Magnetic properties of cherts from the basque-cantabrian basin and surrounding regions: Archeological implications

We present the first rock magnetic study of archeologically-relevant chert samples from the Basque-Cantabrian basin (BCB) and surrounding regions, which was conducted in order to test the usefulness of non-destructive magnetic properties for assessing chert quality, distinguishing source areas, and identifying heated samples in the archeological record. Our results indicate that the studied BCB cherts are diamagnetic and have very low amounts of magnetic minerals. The only exception is the chert of Artxilondo, which has a median positive magnetic susceptibility associated with larger concentrations of magnetic minerals. But even in this case, the magnetic susceptibility is within the lower range of other archeologically-relevant cherts elsewhere, which indicates that the studied BCB cherts can be considered as flint. The similar median values for all magnetic properties, along with their associated large interquartile ranges, indicates that rock magnetic methods are of limited use for sourcing different types of flint except in some specific contexts involving the Artxilondo flint. With regards to the identification of chert heating in the archeological record, our results indicate only a minor magnetic enhancement of BCB natural flint samples upon heating, which we attribute to the low amount of non-silica impurities. In any case, the diamagnetic behavior of most BCB natural flints, along with the local use only of the Artxilondo type, suggests that any flint tool within the core of the BCB with positive magnetic susceptibility values is likely to have been subjected to heating for improving its knapping properties. Further studies are necessary to better identify the type, origin and grain size of magnetic minerals in BCB natural flints, and to apply non-destructive magnetic properties to flint tools in order to identify the use of heat treatment in the BCB archeological record. © 2016 Larrasoaña, Beamud, Olivares, Murelaga, Tarriño, Baceta and Etxebarria.This study was funded by project GUI15/34 of the Universidad del País Vasco.Peer reviewe